This invention is related to piezoelectric materials and, more particularly, to a 0-3 piezoelectric ceramic-polymer composite for hydrophone applications and the like.
Piezoelectricity can be explained as electric polarization produced by mechanical stress in certain substances. Piezoelectric materials, such as lead zirconate titanate (PZT), are used in a wide variety of applications. In hydrophone devices piezoelectric materials detect low frequency acoustic waves passively. Hydrophones are frequently made from single phase PZT. Large hydrostatic piezoelectric charge and voltage coefficients (d.sub.h and g.sub.h) are desired in these devices. Thus, even though the magnitudes of the piezoelectric coefficients d.sub.33 and d.sub.31 of PZT are large, the hydrostatic coefficients d.sub.h and g.sub.h are small, because the d.sub.33 and 2d.sub.31 coefficients are almost equal and opposite in sign, and also, the dielectric constant of PZT is large. The large difference of the acoustic impedance between PZT and water requires impedance matching layers for underwater hydrophone applications.
In order to improve and modify material properties for hydrophone devices, several different types of piezoelectric PZT-polymer composites have been recently investigated utilizing the concept of phase connectivity. It has been found that the electric flux pattern and the mechanical stress distribution together with the resulting physical and piezoelectric properties depend strongly on the manner in which the individual piezoelectric and polymer phases of the diphasic composites are interconnected. Each phase in a composite may be self-connected in zero, one, two, or three dimensions. Thus, a diphasic 2-1 connectivity pattern, for example, has one phase self-connected in two dimensional layers, the other in one dimensional chains or fibers. Below are represented some of the piezoelectric composites with connectivity patterns in which the piezoelectric phase appears first.
0-3 composites: PZT particles suspended in a polymer matrix PA0 1-3 composites: PZT rods aligned in the poling direction held together by a polymer matrix PA0 1-3-0 composites: PZT rods aligned in the poling direction held together by a foamed polymer matrix PA0 3-1 and 3-2 composites: holes drilled in a prepoled PZT block, then the holes filled or covered by polymer. PA0 3--3 composites: lost-wax method using coral as the starting material, or by a fugitive phase method (BURPS process).
The d.sub.h, g.sub.h coefficients and d.sub.h g.sub.h figure of merit of the diphasic composites are significantly improved over single phase PZT due to decoupling of the d.sub.33 and d.sub.31 coefficients and/or the reduction of the dielectric constant.
The piezoelectric ceramic-polymer composites of 1-3, 1-3-0, 3-1, 3-2, and 3-3 connectivities are often expensive and cumbersome to fabricate. The PZT-polymer 0-3 composite is relatively easy and inexpensive to make inasmuch as fabrication involves simply mixing the piezoelectric ceramic particles and polymer, shaping and curing.
However, piezoelectric ceramic powders which are prepared according to the usual methods such as those which are described in the following are accompanied by conspicuous difficulties. The usual piezoelectric ceramic powders are produced by grinding the piezoelectric ceramics prepared by solid-phase reaction such as ceramic materials containing titanium solid solutions of BaTiO.sub.3, PbTiO.sub.3, PbZrO.sub.3 -PbTi0.sub.3, etc. or single crystals such as potassium-sodium-niobate (PSN), etc., using a ball mill, a vibratory mill, etc., and adjusting the resulting powder to a desired size distribution. Composites of these ground powders exhibit inferior piezoelectric properties than what may be expected when considering the piezoelectric nature of the filler materials alone. Moreover, this type of composite material is fragile and hardly lends itself to being shaped due to the lack of flexibility and molded articles made from it are heavy and costly. These disadvantages have led to the result that the point still has not been reached at which the use of compound materials of this type is practical for all intents and purposes.
Extensive research directed at determining the origin of the above-described deterioration of properties has led to the conclusion that structural fractures have appeared in the microcrystals during the comminuting (pulverizing) which is carried out after the solid-phase reaction or the preparing of the single crystals and these fractures lead to the forming of multidomains within the particle fragments. It is almost impossible to force the distorted phases to orient themselves in the same direction as those of the applied polarizing electric field even if the applied voltage is close to the maximum voltage which the composite material can withstand without undergoing dielectric collapse or arcing through. In addition, the electric field which can act effectively on the individual ceramic particles combined with the polymer substance is significantly decreased due to the combining process to a few tenths or a few hundredths of its strength if one takes into account the ratio of the dielectric constants of the polymeric substance to that of the ceramic substance. Therefore, the mixing of the ceramic powders or single crystals with polymeric substances cannot impart piezoelectric properties to the resulting composite materials, to any noteworthy degree.
One attempt to solve the above mentioned disadvantages is disclosed in German Patent No. 2922260 wherein a process for preparing a piezoelectric ceramic powder which has virtually single domain microcrystals is formed. Thus, the piezoelectric ceramic powder is formed by heating the starting powders in a suitable atmosphere so as to undergo a reaction in the solid phase and then cooling the resulting reaction product as desired. The cooling stage is conducted quickly such as by quenching. What has been found is that the orientation in the direction of the applied electrical field is easily achieved with the microcrystals produced by quenching the solid phase because the piezoelectric crystals are not accompanied by structural fractures which cause the production of numerous multiple domains in the microcrystals as a result of stress resulting from the typical pulverizing. Consequently the piezoelectric ceramic powders are able to demonstrate especially high ferroelectric and piezoelectric properties. Among the many types of ferroelectric materials disclosed by this patent, lead titanate and solid solutions containing lead titanate as the main component, for example, PbTiO.sub.3 -BiFeO.sub.3 are disclosed.